AV referendum: Where parties stand
A referendum on the Westminster voting system is being held on 5 May. Voters will be asked whether they want to replace the existing first-past-the-post system used to elect MPs in favour of the alternative vote system. Gavin Stamp examines where each of the parties stand.
The Conservatives agreed to a referendum being held as part of the coalition deal with the Liberal Democrats.
But most Tory MPs, including Prime Minister David Cameron, are campaigning against any change.
They believe first-past-the-post is a tried and tested system which generally provides stable government and maintains the direct link between an MP and their constituency.
During the 2010 general election campaign, many Tories warned changes to the electoral system could lead to permanent coalitions.
More recently, it has been suggested that some Tory MPs are "relaxed" about the possibility of losing the referendum, thinking it would not damage their chances of winning a majority in future.
But most Conservative activists in the country are thought to be strongly opposed to any change.
Labour leader Ed Miliband is supporting the Yes campaign for changing the electoral system, believing it is fairer than the current situation and good for democracy and accountability.
Other senior figures such as Alan Johnson also support AV, but the party as a whole is divided on the issue, with more than 100 Labour MPs saying they oppose such a change.
Several current shadow cabinet members, including John Healey and Caroline Flint, as well former ministers such as David Blunkett, Lord Prescott and Margaret Beckett, are actively campaigning against AV.
Gordon Brown offered the Lib Dems a referendum on voting reform as part of their own coalition negotiations after May's election.
The party flirted with voting reform as an issue before coming to power in 1997, asking the late Lord Jenkins to head a review into the subject.
But his conclusions in 1998 were largely ignored and critics say Labour only returned to the issue when the party looked set to leave power.
THE LIBERAL DEMOCRATS
The Lib Dems are pushing strongly for a yes vote, having launched their own pro-change "fair votes" campaign at their party conference in September.
The party has long called for an overhaul of the electoral system, saying first-past-the-post does not reflect the will of the people, too many votes simply do not count and smaller parties are penalised.
Under the current system, they argue, Labour won power in 2005 with 35% of the popular vote.
AV is not the party's preferred choice, as it has historically wanted a single transferable vote system. In this system, voters rank candidates in order of preference and more than one MP is elected from each constituency.
But its MPs see AV as fairer than the current system and a step forward.
However, what will happen in the event of the party losing the poll, is unclear. Lib Dem ministers have said this would not threaten the future of the coalition but some backbenchers are less sure.
After some consideration, the Scottish National Party has confirmed that it supports a Yes vote in the forthcoming AV referendum. It said the change represented a step towards a more democratic system.
However, its preferred choice of voting system is Single Transferable Vote (STV) for all elections.
The party said: "Faced with a choice between the fundamentally unfair first-past-the-post system and a step towards a more democratic system, the SNP's National Executive has agreed that the SNP will support a Yes vote in the AV referendum."
The SNP remains unhappy that the AV referendum, being held on 5 May, is on the same day as the Scottish Parliament election, saying this will distract from debate about Scotland's future and multiple ballots may confuse voters.
As supporters of changing the electoral system, Plaid Cymru are backing the switch to AV - although they actually support the single transferable vote system.
But the party says they will not be taking an "active" role in the campaign as they are focused on elections to the National Assembly of Wales on the same day.
The party objects to the referendum being held on 5 May, saying there was not enough consultation with the authorities in Wales about combining the two polls and this has "huge consequences" for Welsh democracy.
NORTHERN IRELAND PARTIES
The referendum date caused some concern in Northern Ireland, where assembly elections will also be held on 5 May.
The Democratic Unionists (DUP) and Social Democratic and Labour Party (SDLP) both signed a letter calling on MPs to be given more time to debate the proposals - which would have been likely to delay the poll date until the autumn.
Since the campaign has got under way, the SDLP has swung behind a "Yes" vote, arguing that AV is "a clear step forward". Sinn Fein says it will also be telling its supporters to vote "yes". The cross community Alliance Party, which is a sister party of the Liberal Democrats, is also in favour of AV.
The DUP, which has the most MPs of any Northern Ireland party, is backing the retention of First Past The Post.
Some analysts argued that the smaller Ulster Unionist Party (UUP) would have benefited from AV in last year's election. But despite that the UUP, which has had links to the Conservatives, is also in favour of retaining the current system.
THE GREEN PARTY
The Green Party of England and Wales will be supporting the "yes" campaign although AV is not their preferred system.
The party supports the additional member system and its leader Caroline Lucas - elected as an MP under the first-past-the post system in May - wants voters to be able to choose between a range of different systems in the referendum.
But the Greens have urged their members to put their "full weight" behind the pro-change campaign.
"A Yes vote would bring a step in the right direction and demonstrate an appetite for change," says deputy leader Adrian Ramsay. "Greens and others who want a fair, inclusive proportional way of voting will then continue to campaign for further reform."
Under the additional member system, each voter typically gets two votes - one for an individual, and one for a party. The exact proportion of constituency representatives and list representatives varies from country to country.
The Green Party of Scotland, which is a separate entity, also backs AV but their preferred system is Single Transferable Vote.
In contrast to its counterparts in England, Wales and Scotland, the Green Party in Northern Ireland is opposed to AV, arguing that the system is inferior to STV and could bolster sectarian voting patterns.
UK INDEPENDENCE PARTY
UKIP's central policy making committee has decided to campaign for a yes vote in the referendum although some of its MEPs are believed to be sceptical.
Leader Nigel Farage says first-past-the-post is a "nightmare" for the party although he has acknowledged AV is likely to make "little difference" to the party's electoral fortunes
The party failed to win a seat in May's general election. But under the regional PR system used for European elections in 2009, UKIP came second in terms of the national vote.
Many in the party maintain that the separate AV Plus system is the system likely to see "plenty of" UKIP MPs elected to Westminster.
Under this system, recommended by Lord Jenkins in 1998, most MPs would be elected via constituencies under AV. But about 10% would be chosen from party lists in a separate vote from designated regions.
The BNP says it backs the decision to hold a referendum but will campaign against as they regard AV as "an even more unfair version" of first-past-the-post.
The party supports a system known as party list proportional representation, which is used in England, Scotland and Wales for elections to the European Parliament.
Representatives are elected on the basis on how many votes parties get in different regions, using a quota system. Parties have to get a certain share of the vote to qualify for any seats.
The BNP says this threshold should be set at a "minimum level".
The English Democrats support the single transferable vote system but say they are "fairly enthusiastic" about AV as a "first step" towards more fundamental reform.
"AV will open people's eyes to other options they could vote for rather than following traditional, tribal voting patterns," says chairman Robin Tilbrook. "That must be good for us and better for democracy."
However, he says the way the AV process is conducted is all important.
Should no-one win a majority, he says only the candidate with the least support should be eliminated - and their votes shared out - in each round. Knocking out more than one candidate at a time would "choke off" support for smaller parties.
CHRISTIAN PEOPLES ALLIANCE
The CPA's ruling body voted last year to support the yes campaign in the referendum.
While the AV system has "problems", the party says it would be a "first step" towards getting a truly proportional system for electing MPs.
"In hundreds of parliamentary constituencies, there are thousands of disenfranchised voters whose party preferences are simply discounted by the first-past-the-post system," says leader Alan Craig.
"The same party wins every time, even with a minority of voters backing them. This is bad for democracy, especially as commitment to the big parties is withering away."
Respect says first-past-the-post is "antiquated and undemocratic" and distorts election results in the favour of the Conservatives and Labour.
But it will campaign against AV as it wants a "fully" proportional system, in which the number of MPs is "directly equal" to votes cast.
While supporting political reform and greater use of referendums in general, the Jury Team says the referendum is flawed and AV "exaggerates results".
Policymakers should revisit the findings of the 1998 Jenkins Commission as a prelude to a "two-stage" referendum, its founder Sir Paul Judge says. Voters should choose an alternative to the current system before deciding between the two.
"It is a wrong use of a referendum and a political stitch-up," he says. "It does not increase the independence of MPs and does not necessarily give a better reflection of votes in Parliament."
The Communist Party is opposed to any switch to AV, arguing the system is complex and will reinforce what it says is an already "limited" system of democracy.
It supports the single transferable system as the only way to address the current "democratic deficit". It says the poll should be treated as a referendum on the coalition.